This invention pertains to emissions reduction from signals along communications lines. More particularly, this invention relates to the routing of traces on a printed circuit (PC) board to decrease emissions that are radiated from high speed differential signals on the PC board.
Differential twisted pair cabling has previously been implemented to help reduce radio frequency interference (RFI) emissions, or more broadly, electromagnetic field emissions resulting from transmission of signals on signal cables. In order to reduce RFI, a cancelling effect is generated by twisting together a pair of wires. Such result is relatively easy to achieve with a pair of cables. However, successful implementation of signal paths on a printed circuit (PC) board in a manner that reduces RFI using conductive traces is more difficult to realize. Accordingly, further improvements are needed in order to maximize electromagnetic field and RFI emissions cancellation.
One PC board implementation for routing differential signals on a printed circuit (PC) board consists of routing a pair of conductive traces close together and in parallel at a minimum possible adjacent spacing. Additionally, the differential pair of conductive traces are sometimes ground guarded using conductive ground traces on both of the pair of conductive traces. However, an undesirable current loop results from separation of the two conductive traces.
More particularly, such a parallel pair of conductive traces will emit RFI emissions, and more broadly, electromagnetic field emissions. Such electromagnetic (E) field emissions are generated from the differential loop, and are proportional to:
xe2x80x83Eƒ2*A*I
where f is the frequency of the signal, A is the area of the loop that is created by the separation of the two conductive traces, and I is the current within the two conductive traces.
As an example, the effective radiating area of one exemplary differential loop, created by a 6-inch differential conductive trace run that is separated by 0.006 inches, is 0.036 inches-2. For cases of relatively high frequency signal transmission and/or high current levels, such a relatively small area can create significant levels of radiation.
Accordingly, improvements are needed to more effectively reduce RFI emissions, as well as electromagnetic field (E-field) emissions, on conductive traces of printed circuit (PC) boards.
A differential twisted-pair cabling implementation on a printed circuit (PC) board maximizes RFI and E-field emissions cancellation. Additionally, trace impedance is also adjustable to facilitate matching with an associated cable impedance by providing for a geometry that enables adjustment of conductive trace width on the PC board.
According to one aspect, a printed circuit board includes a first helical conductive trace and a second helical conductive trace. The first helical conductive trace extends generally along a longitudinal axis of a layer of the printed circuit board. The first helical conductive trace is provided successively: along a first surface, from the first surface through the layer to a second surface, along the second surface, and from the second surface through the layer to the first surface. The second helical conductive trace extends generally along the longitudinal axis of the layer of the printed circuit board. The second helical conductive trace is provided successively: along the second surface, from the second surface through the layer to the first surface, along the first surface, and from the first surface through the layer to the second surface.
According to another aspect, a printed circuit board includes a first conductive trace and a second conductive trace. The first conductive trace extends along a longitudinal axis of a layer of the printed circuit board provided in series along a first surface, from the first surface through the layer to a second surface, along the second surface, and from the second surface through the layer to the first surface. The second conductive trace extends along the longitudinal axis of the layer of the printed circuit board provided in series along the second surface, from the second surface through the layer to the first surface, along the first surface, and from the first surface through the layer to the second surface. The first conductive trace and the second conductive trace are inter-nested and both extend in one of a clockwise and a counter-clockwise helical pattern about a substantially common longitudinal axis of the printed circuit board.
According to yet another aspect, a method is provided for reducing radio frequency interference emissions from conductive traces on a printed circuit (PC) board. The method includes: providing a printed circuit board; forming a first conductive trace for carrying a signal generally along a longitudinal axis of a layer of the printed circuit board provided in series along a first surface, from the first surface through the layer to a second surface, along the second surface, and from the second surface through the layer to the first surface; and forming a second conductive trace for carrying a return signal generally along the longitudinal axis of the layer of the printed circuit board provided in series along the second surface, from the second surface through the layer to the first surface, along the first surface, and from the first surface through the layer to the second surface.